diff --git a/internal/security/flock.go b/internal/security/flock.go
index 78552a9..2d18ad9 100644
--- a/internal/security/flock.go
+++ b/internal/security/flock.go
@@ -16,6 +16,11 @@ func Flock(path string) (release func(), err error) {
 	if err != nil {
 		return nil, err
 	}
+	// REQ-139 / F15: tighten pre-existing looser perms to 0600.
+	// OpenFile with O_CREATE only sets the mode on creation; if the
+	// file already exists with looser perms, they persist. Chmod
+	// ensures 0600 regardless.
+	_ = os.Chmod(path, 0o600)
 	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
 		f.Close()
 		return nil, err